Wireless network topologies can be controlled by
 •  Data rate
 •  Channel allocation: different devices communicate at different channels
      In some cases, there is a channel dedicated for the control (management) and 
 message exchange
 •  Transmission power (power control)
 •  Carrier sense threshold
 •  Directional antennas
 •  Cognitive intelligent radios & software defined radios
 •  Node placement
 •  Different network architectures/deployments (e.g., mesh networks, 
    infrastructure-based, ad hoc)
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                   IEEE 802.11 Rate Adaptation
      • The 802.11 a/b/g/n standards allow the use of multiple 
         transmission rates
          – 802.11b, 4 rate options (1,2,5.5,11Mbps)
          – 802.11a, 8 rate options (6,9,12,18,24,36,48,54 Mbps)
          – 802.11g, 12 rate options (11a set + 11b set)
      • The method to select the transmission rate in real time is 
         called “Rate Adaptation”
      • Rate adaptation is important yet unspecified by the 
         802.11 standards
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   IEEE 802.11 Rate Adaptation
 •  IEEE802.11b
     11, 5.5, 2, 1 Mbps
 •  IEEE802.11a
     6, 9, 12, 18, 24, 36, 48, 54 Mbps
 •  IEEE802.11g
       802.11b rates + 802.11a rates
 •  Most of existing wireless radios are able to support multiple 
    transmission rates by a combination of different modulation and 
    coding rates
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   IEEE802.11 Bitrate Adaptation
 • When a sender misses 2 consecutive ACK
 Drops sending rate by changing modulation or 
   channel coding method
 •  When 10 ACKs are received successfully
  Transmission rate is upgraded to the next higher 
   data rate
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                Rate adaptation example
                                          Signal becomes weaker
                             54Mbps
                          Signal is good   12Mbps
                     Sender             Receiver
      • Ideally, the transmission rate should be adjusted 
         according to the channel condition
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